Aircraft main engines not only provide propulsion for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main engines may not be capable of supplying power. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may additionally be used to start the main propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustor, a power turbine, and a compressor. During operation of the APU, compressor draws in ambient air, compresses it, and supplies compressed air to the combustor. The combustor receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy compressed air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive the compressor. In some instances, an APU may additionally include a starter-generator, which may either drive the turbine or be driven by the turbine, via the turbine output shaft. Some APUs additionally include a bleed air port between the compressor section and the turbine section. The bleed air port allows some of the compressed air from the compressor section to be diverted away from the turbine section, and used for other functions such as, for example, main engine starting air, environmental control, and/or cabin pressure control.
Although most APUs, such as the one generally described above, are robust, safe, and generally reliable, some APUs do suffer certain drawbacks. For example, when some APUs are operated at part power, the surge margin of the APU compressor, or at least one or more stages of the compressor, can be reduced. At part power conditions, the compressor flow rate is reduced, but the compressor is sized to deliver the required high-speed flow rate. When the compressor is operated at reduced speed and power conditions (e.g., at specific-fuel-consumption (SFC)-critical, part-speed, part-power conditions), the impeller blade leading edge will be operating at high incidence angles. This dramatically reduces compressor efficiency and surge margin at part power.
One approach to improving SFC-critical, part-speed, part-power surge margin and overall efficiency is to include a plurality of vanes (or airfoils) within the compressor shroud. Such a vaned shroud is disclosed in U.S. Pat. No. 5,277,541, which is assigned to the assignee of the present invention, and achieves the function of a variable flow capacity impeller. The disclosed vaned shroud may be desirable because it is passive in function. It also provides significant surge margin increase, eliminates the need for surge bleed and/or variable geometries, and lowers recirculation losses as compared to a conventional ported shroud design. However, the disclosed vaned shroud does not include various features that further improve overall surge margin and efficiency.
Hence, there is a need for an vaned shroud that further improves the surge margin, and overall operational efficiency, of a compressor as compared to presently known vaned shrouds. The present invention addresses one or more of these needs.